Acousto-optic filter having means for damping acoustic resonances and stray light

ABSTRACT

Acoustic resonances in the acoustically excited photoelastic optically birefringent crystal of an acousto-optic filter are damped by coating the side of the photoelastic crystal with an acoustic absorbing medium, such as tungsten powder loaded epoxy, whereby undesired acoustic resonances are damped. The acoustic impedance of the coating preferably matches the acoustic impedance of the crystal. The outside surface of the crystal is preferably cleaned and roughened to assure an intimate bond between the absorbing material and the crystal medium.

350-372 SR OR 3n72$v250 P United St: X 49 W4 1 1 29,250 Kusters et al.1451 Apr. 24, 1973 [5 1 ACOUSTO-OPTIC FILTER HAVING 3,614,201 10/1971Biazzo et al. ...350/l6l MEANS FOR DAMPING ACOUSTIC 3,637,288 1/1972 Seidel 2.392.350 1/1946 W1llard. ..350/l6l [75] Inventors: John A.Kusters, San Jose; David OTHER PUBLICATIONS 3' 3 g i gf fi Harris etal., CaMoO Electronically Tunable Optiol 0 ac o cal Filter App. Phys.Lett. Vol. 17, No. 5 (Sept. 1 l970) PP. 223-225. [73] Ass1gnee:Hewlett-Packard Company, Palo Alto Cahf' Primary ExaminerJohn K. Corbin22 Filed; J l 23, 1971 Attorney-Roland l. Griffin Acoustic resonances inthe acoustically excited [52] U.S- Cl ..350/149, 350/161 photoelasticoptically birefringent crystal of an [5 l 1 III. C) 1/24 acousto opticfilter are damped y coating the side of [58] Fleld of Search ..350/149,150, 157, the photoelastic Crysal with an acoustic absorbing 350/160 161medium, such as tungsten powder loaded epoxy, whereby undesired acousticresonances are damped. [56] References and The acoustic impedance of thecoating preferably UNITED STATES PATENTS matches the acoustic impedanceof the crystal. The outs1de surface of the crystal 15 preferably cleanedand 3,572,897 3/197! Bousky ..350/l50 roughened to assure an intimatebond between the ab- 3,653,743 4/l972 Kiefer et al... ..350/l6l so -hingmaterial and the crystal medium. 3 632 l93 l/l972 Kusters ...350/l493,644,0l5 2/l972 Hearn ..350/l6l 2 Claims, 4 Drawing Figures PatentedApril 24, 1973 3,729,250

2 L VERTICAL 1e LINEAR 1 X Y g2 POLARIZER 3 14 LIGHT 4 k l SOURCE '1; 3

TUNABLE R.F. SOURCE igure 1 X INVENTORS JOHN A. KUSTERS DAVID A. WILSONLAURENCE M. HUBBY,JR.

ATTORN ACOUSTO-OPTIC FILTER HAVING MEANS FOR DAMPING ACOUSTIC RESONANCESAND STRAY LIGHT DESCRIPTION OF THE PRIOR ART wave in an opticallyanisotropic medium, such as a 0 photoelastic birefringent crystal, toshift the polarization of the polarized input light beam at a selectedbandpass optical frequency from the first polarization to a secondorthogonal polarization. The diffracted light was polarization analyzedto separate the light of the second polarization from light of the firstpolarization. The bandpass of the filter was electronically tunable byvarying the frequency of the acoustic wave within the birefringentcrystal.

Such an acousto-optic filter is disclosed in an article titledAcoustoOptic Tunable Filter appearing in the Journal of the OpticalSociety of America, Volume 59, No. 6 of June of 1969, pages 744-747, andin an article titled Electronically Tunable Acousto-Optic Filterappearing in the Applied Physics Letters, Volume 15, No. 10, of Nov.1969, pages 325 and 326. One of the problems encountered with this priorart acoustooptic filter, which is of the type wherein the light beam isprojected through the crystal, is that reflection of acoustic waves fromthe output end of the crystal back to the input end sets up standingacoustic waves or acoustic resonances within the photoelasticbirefringent crystal. These acoustic resonances introduce ripples in theoptical bandpass characteristic of the filter, and it is desired toreduce or eliminate these ripples.

SUMMARY OF THE PRESENT INVENTION The principal object of the presentinvention is the provision of an acousto-optic filter having means fordamping acoustic resonances.

In one feature of the present invention, an acoustic absorbing medium iscoupled to the photoelastic optically birefringent crystal for absorbingacoustic wave energy reflected from the output end of the crystal,whereby undesired acoustic resonances are damped.

In another feature of the present invention, the acoustic absorbingmedium comprises a coating of acoustic absorbing material on the sidesof the photoelastic crystal.

In another feature of the present invention, the acoustic absorbingmedium which is coupled to the photoelastic crystal comprises anadhesive loaded with particles of a material having a density which issubstantially greater than the density of the adhesive.

In another feature of the present invention, the acoustic absorbingmedium coated onto the sides of the photoelastic crystal comprises anepoxy adhesive loaded with particles of tungsten.

In another feature of the present invention, the acoustic absorbingmedium coupled to the photoelastic birefringent crystal is made of amaterial having a product of its density and acoustic velocitiesapproximately equal to the product of the acoustic velocity and densityof the photoelastic crystal material to which it is coupled, whereby theacoustic impedance of the acoustic absorbing medium is substantiallymatched to the acoustic impedance of the crystal to obtain maximumtransfer of acoustic energy to the absorbing 7 medium.

Other features and advantages of the present invention will becomeapparent upon a perusal of the following specification taken inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic line diagram,partly in block diagram form, of an acousto-optic filter incorporatingfeatures of the present invention.

FIG. 2 is a side elevational view of a portion of the structure of FIG.1 taken along line 2-2 in the direction of the arrows.

FIG. 3 is an enlarged sectional view of the structure of FIG. 1 takenalong line 3-in the direction of the arrows.

FIG. 4 is a plotof optical beam intensity I versus optical frequency1",, depicting the bandpass characteristics of the acousto-optic filterof FIG. 1 with and without acoustic resonances within the photoelasticbirefringent crystal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIGS. 1 and 2,there is shown an acousto-optic filter l incorporating features of thepresent invention. Acousto-optic filter l is substantially the same asthat disclosed in the aforecited Journal of the Optical Society ofAmerica. More specifically, the acousto-optic filter 1 includes a lightsource 2 which projects a beam of light 3 through a vertical linearpolarizer 4 into an optically anisotropic photoelasitc medium 5, such asa crystal of LiNbO,, PbMoO CaMoO or quartz. The beam of light isdirected against an input face 6 of the crystal 5 at such an angle thatthe light beam is diffracted through the crystal 5 parallel with the Yaxis thereof between the end faces 6 and 7. The Y axis is notnecessarily the crystalline Y axis.

The light source 2 may be of any type. For example, it may be a coherentlight source, such as a laser, or it may be a broadband light'sourcehaving a uniform spectral power density, such as a white light source.The input vertical polarizer 4 serves to pass only that light from thesource 2 which is polarized in the vertical direction, i.e., the Zdirection, to provide a polarized input light beam 3. The input lightbeam enters input face 6 of the crystal 5 and travels generally alongthe longitudinal axis (Y axis) of the crystal 5 and passes out theopposite end face 7 of the crystal 5 as an output beam 3.

For a particular combination of light wave and acoustic wavefrequencies, there is found to be a strong interaction between the lightwave and the acoustic wave in which the acoustic wave diffracts aportion of the light wave from the polarization orientation of the inputbeam 3 into an orthogonal polarization. This yields a narrow band oflight of orthogonal polarization which is separated from the output beam3 by means of a polarization analyzer 14, such as a Rochon or Glan-Taylor prism.

Polarization analyzer 14 is oriented to pass that por tion of the outputbeam 3 having a cross-polarization to the polarization of the inputpolarizer 4, namely, in the horizontal direction, as output beam 15.Output beam then comprises a bandpass output characteristic. Thatportion of the light of the output light beam 3 which has the samepolarization as the input beam is reflected as a second output beam 16via the polarization analyzing prism 14. Output beam 16 comprises allthe input vertically polarized light minus that portion or notch of thelight which has been converted from the vertical polarization to thehorizontal polarization. Thus, output beam 16 has a band reject or notchband characteristic.

That light which is diffracted from the input polarization to thecrossed or orthogonal polarization has an optical frequency related tothe frequency of the acoustic wave f,, by the following relation:

where c/ V is the ratio of the light velocity in vacuum to the acousticvelocity in the medium, and lAnl is the birefringence of the crystal 5.

In a typical example employing a lithium niobate crystal 5, theacousto-optic filter 1 is tunable from 7,000 to 5,500 A by changing theacoustic drive frequency from 750 to 1,050 MHz. A passband of less than2 A is obtained for output beam 15 when a crystal 5 centimeters long isemployed.

In the prior art, the photoelastic birefringent crystal 5 had its inputface 6 and output face 7 out such that the normals to the cut faces bothlie in the YZ plane of the crystal and such that the acoustic waveenergy S reaching the output face 7 was reflected toward the side wallsof the crystal to minimize'the possibility of setting up unwantedacoustic resonances within the crystal. However, it has been found thatsuch acoustic resonances are not entirely eliminated in this manner.

Accordingly, in the present invention, an acoustic absorbing medium iscoupled to the sides of the crystal 5, as by coating the crystal 5 withan acoustic absorbing material 18 see FIG. 3). The acoustic absorbingmaterial 18 preferably is bonded or coupled to the crystal 5 via anintimate bond to assure sufficient coupling between the coating 18 andthe crystal 5. In addition, the coating 18 preferably has an acousticimpedance approximately equal to the acoustic impedance of the crystal 5such that maximum transfer of acoustic power is obtained from thecrystal to the acoustic absorbing layer 18. More particularly, theproduct of density and acoustic velocity in the crystal 5 should beapproximately equal to the product of density and acoustic velocity inthe coating 18.

In addition, the end faces 6 and 7 of the crystal 5 are preferably cutwith the normals to the faces lying in orthogonal planes such as the z-yplane and x-y plane, respectively, to reflect acoustic waves inorthogonal planes. Moreover, the output face 7 is preferably cut atBrewsters angle to the Y axis to minimize reflection of the optical beam3.

In a typical example, the acoustic absorbing coating 18 comprises arelatively low viscosity acoustically lossy epoxy adhesive, i.e., aviscosity between and 500 cp. Such an epoxy is marketed by EpoxyTechnology, Inc. of Watertown, Mass. as Epo-Tek 305. This epoxy adhesiveis loaded in approximately equal volume portions with a relatively highdensity material such as tungsten powder. The powder is less than 2 milsin diameter and preferably in the micron size range.

The loaded epoxy is coated onto the cleaned and roughened translucent ordiffusely reflecting side surfaces of the crystal 5 to assure anintimate bond between the adhesive and the crystal. The acousticabsorbing coating 18 is coated to a thickness of approximately 0.5millimeter.

Another advantage of using tungsten powder as the acoustic loadingmaterial 18 is that tungsten powder absorbs light. In any realacousto-optic filter of this type there is always stray light presentwhich results from scatter at the various surfaces of the inputpolarizer, input prism, and input end of the birefringent medium, eitherdue to imperfect surface quality or imperfect coatings, or both. Thisstray light is, in general, depolarized and scattered most strongly inthe forward directions. Such light will, if directed through the outputpupil of the acousto-optic filter, degrade the overall signal'to-noiseratio of the device and hence is undesirable. Heretofore, in acoustooptic filters much of this stray light was indeed directed through theoutput pupil of the device by reflection and/or diffuse scattering atthe side walls of the birefringent medium. The method of the presentinvention overcomes this problem to a large extent because a transparentepoxy cement can be used as a carrier which has an index of refractionmuch closer than that of air to that of the birefringent medium, causinga substantial amount of the stray light which strikes the side walls ofthe birefringent medium and would thus normally be reflected and/orrescattered to be coupled out of the birefringent medium and absorbed bythe tungsten powder.

We claim:

1. An acousto-optic filter comprising a photoelastic birefringentcrystal, means for directing a polarized beam of light through saidphotoelastic birefringent crystal, means for directing an acoustic wavecollinearly with the polarized beam of light through said photoelasticbirefringent crystal to shift a portion of the polarized beam of lightfrom a first polarization to an orthogonal second polarization, and asingle coating comprising a mixture of acoustic and light energyabsorbing material on the sides of said photoelastic birefringentcrystal to damp undesired acoustic resonances and stray light.

2. An acousto-optic filter as in claim 1 wherein said coating ofacoustic and light energy absorbing material comprises an epoxy adhesiveloaded with particles of tungsten.

* I I t

1. An acousto-optic filter comprising a photoelastic birefringentcrystal, means for directing a polarized beam of light through saidphotoelastic birefringent crystal, means for directing an acoustic wavecollinearly with the polarized beam of light through said photoelasticbirefringent crystal to shift a portion of the polarized beam of lightfrom a first polarization to an orthogonal second polarization, and asingle coating comprising a mixture of acoustic and light energyabsorbing material on the sides of said photoelastic birefringentcrystal to damp undesired acoustic resonances and stray light.
 2. Anacousto-optic filter as in claim 1 wherein said coating of acoustic andlight energy absorbing material comprises an epoxy adhesive loaded withparticles of tungsten.